Electric power conversion devices, such as transformers and power supplies, are indispensable products in modern society. Various types of electric power conversion devices are widely used in daily living. However, the working principle thereof is no more than to modulate voltage and rectify current. The fabrication of power conversion devices has been a mature technology, but they can still advance in cooperation with semiconductor technologies.
Inside a power supply, there is a transformer to convert commercial power into stable direct current (DC) power. The transformer utilizes the electromagnetic induction inside two coils to convert electric energy. A coil is equivalent to an inductor in the circuit theory. When a current flows through the coil of a transformer, an induced current is generated. According to the Lenz's law, the induced current flowing through the coil will generate an extra magnetic field, and an opposite induced magnetic field is then generated to resist the extra magnetic field. Thus, the change of the current in a coil will generate an opposite induced current, i.e. the coil of an inductor is apt to resist the change of the current in the coil. In other words, the current in a coil has inertia to resist change. The abovementioned characteristic of coils results in some problems of the conventional transformers. Refer to FIG. 1 a block diagram schematically showing the circuit of a conventional power supply and FIG. 2 a diagram schematically showing the waveforms of a conventional power supply. The conventional power supply commonly has a pulse control unit 3 to control a switch unit 4; the conduction cycle of the switch unit 4 is used to regulate the current flowing through the coil of the primary side of a transformer and make the output terminals Vout of the secondary side of the transformer able to provide a stable voltage for loads. A voltage feedback unit 5 provides a feedback signal to the pulse control unit 3. When the load increases, the fallen output voltage will make the pulse control unit 3 prolong the conduction cycle of the switch unit 4 to increase the current of the primary side of the transformer so that the secondary side can provide more electric energy. Thereby, when there is a heavy load, the balanced current of the coil is boosted. In addition to the inertia of keeping current, the current-rising rate is greater than the current-falling rate in the coil; therefore, the average current is hard to fall to the original value. If the load constantly varies for a period of time, the suddenly increasing load will exaggerate the excitation current Im to exceed the saturated excitation value 7 of the coil. Thus, the transformer is saturated, and a surge current 8 is generated. The IC chip used by the conventional pulse control unit 3 usually has an over-current protection function. When the surge current 8 appears, the pulse control unit 3 will turn off the switch unit 4 to eliminate the surge current 8. However, via the mutual induction between the coils of the transformer, the sudden appearance of the surge current 8 will apply a very great surge voltage 81 to the switch unit 4, which may seriously damage the power supply Refer to FIG. 1 and FIG. 2 again. I_Lo represents the current of the output terminals of the secondary side. The falling of I_LO signifies that the load decreases. When I_Lo falls to below zero, it signifies that the redundant energy is being recycled. The rising of I_Lo signifies that the load increases. Vds represents the voltage difference between two terminals of the SW1 of the switch unit 4. The excitation current Im is the current flowing through the primary coil. The duty cycle signal Va is the output of the pulse control unit 3. The pulse control unit 3 outputs the duty signal Va to control the conductive activities between the switches SW1 and SW2. Thus, the excitation current Im has an excitation cycle wherein Im increases and a demagnetization cycle wherein Im decreases. When the load constantly changes, the DC level of the excitation current Im will also constantly uprise because of the abovementioned characteristic of the coil. Further, the resistance to the change of the coil's current results in that the excitation current falls very slowly. Besides, the conventional power supply does not have the mechanism to make the excitation current Im fall to the original level. Thus, the excitation current Im will approach the saturation state in the cycles wherein the load varies more obviously, and the elements of the power supply is likely to be damaged by the surge voltage 81.